The cantilever fixed partial denture-A literature reviewRaphael Himmel, Raphael Pilo, David Assif, Israel Aviv,(J PROSTHET DENT 1992;67:484-7.)

• The cantilever fixed partial denture (FPD) is a restoration with one or more abutments at one end and unsupported at the other end.’

FORCES AND STRESS DISTRIBUTION• Wright and Yettram, using the finite element

method of stress analysis in a two-dimensional form, demonstrated that a nonaxial force applied on a single tooth elevated forces in the periodontal membrane compared with an axial load on the same tooth.

• The forces were magnified as the loading on the tooth became more oblique.

• Wright and Yettram also confirmed that the force increased on the periodontal membrane of a single tooth abutment with a cantilever if the point load was direct to the free end of the extension.

• If two abutment teeth with a cantilever were splinted, both teeth resisted the rotating movement, and the force on the periodontal membrane for both teeth was diminished.

• Henderson et al used a practical model and a laboratory model of a three-abutment posterior FPD with strain gauges.

• In both models, forces transmitted to the abutments through the cantilevered pontics were resisted by rotational and tilting movements of the abutments but not parallel to the vertical axis of the abutment roots.

• More than 50 % of the force applied to the cantilever pontic was absorbed by the abutment nearest the cantilever pontic, but the addition of abutment teeth lessened the force on the distal abutment.

• The mechanoreceptor mechanism of the periodontal membrane was emphasized by Randow and Glantz.

• They discovered that there was a definite difference in the biomechanic reactions on cantilever loading between vital and nonvital teeth.

• The tolerable loading levels in nonvital teeth were twice that of vital teeth.

• Nevertheless, the vital and nonvital teeth had the same level of tolerable loading when the abutments were anesthesized.

• They concluded that the vital teeth with optimal bone support had a more efficient form of mechanoreceptor function at lower degrees of bending than nonvital teeth.

• This elevated response may also explain the greater mechanical failure associated with endodontically treated abutment teeth.

BIOMECHANICAL CONSIDERATIONS• The mechanics of the cantilever FPD require the

abutment adjacent to the replacement tooth to possess suitable periodontal support, because the greatest functional stress is directed to the nearest abutment.

• Conversely, the abutment tooth farthest from the pontic should be extremely retentive to resist the vertical dislodgement imparted by the cantilevered pontic.

• Ewing cited the following criteria crucial for success in evaluating the cantilever principle:

1. acceptable periodontal attachment and alveolar support,

2. favorable root length and shape, 3. sufficient crown length, and 4. a harmonious arch-to-arch and tooth-to-tooth

relationship.

• Wright and Yettram concluded that a cantilever FPD should have at least two abutments and not replace more than one tooth.

• When a cantilevered pontic is placed posteriorly, additional abutments may be included to withstand the strong forces of the muscles of mastication.’

• Echardson estimated a maximal force of 700 N acting on the cantilever and believed that such a force could destroy the FPD adjacent to the cantilever.

• However, Lundgren and Laurel reported that the greatest individual local force was 150 N if all stresses were actively concentrated at the distal cantilever unit. They claimed that such forces were unlikely to occur in natural function unless the cantilevers are subjected to premature contacts of inordinate and nonclinical size.

• Glantz et al illustrated that the greatest strain in cantilevered FPDs was recorded mesial to the most distal retainer, identifying this region as vulnerable.

• In extensive oral rehabilitation with numerous edentulous spaces, the distal retainer should be designed as complete crowns with near parallel walls on the tooth preparation to ensure retention.

• Failures were common when the distal abutments were nonvital, possibly because of systematic, excessive strain directed to the nonvital teeth.

• Because these progressive types of fractures are time-dependent, fatigue may be a crucial consideration.

• Most restorative fractures occurred mesial to the most distal cantilevered abutments.

• The maximal strength of most luting cements is compressive, the minimal strength is tensile, and the shear strength has an interval value.

• Apically directed forces on the cantilever direct tensile forces to the cement of the retainer farthest from the cantilever.

• The tooth preparation of the abutment should have adequate vertical length and parallel sides to resist tensile shear forces.

• The cement should also possess maximal tensile and shear strength.

• The prognosis will improve if:1. Abutment teeth have long roots and acceptable

alveolar support.2. Tooth preparations for abutments have adequate

length and parallel-sided walls.3. Vital abutment teeth are included.4. The number of abutments are increased but the

number of pontics are decreased.5. The retainers are bilateral and the pontics are

unilateral.

6. The occlusion is well equilibrated and harmonious.7. Complete crowns are of the veneer type.8. Connectors of the distal abutment tooth adjacent to the pontic are sturdy.9. The patient’s hygiene is exemplary.10. The restorative material used for the FPD is extremely rigid.11. The cement has a high tensile strength.

CLINICAL APPLICATIONS• Budtz-Jorgensen et al reported on 27 patients treated with

maxillary complete dentures and distally extension cantilever FPDs.

• All 27 patients, including those who formerly adapted well to the RPD, expressed a pronounced improvement in mastication and in the stability of the complete maxillary denture.

• After 2 years, all FPDs were intact and the abutments asymptomatic.

• Budtz-Jorgenson and Isidor compared 27 patients treated with mandibular distal extension cantilever FPDs and 26 patients with mandibular RPDs, and concluded that symptoms of mandibular dysfunction were significantly aggravated in the RPD group during the 2-year study. The occlusion was satisfactory after 2 years in 90% of the FPD group but in only 76% of the RPD group.

• The cantilever FPDs require at least two abutment teeth.

• The only documented exception permitting a single abutment is the replacement of a maxillary lateral incisor with the canine as an abutment.

• Although a healthy canine commonly exhibits adequate periodontal support, the pontic, without a mesial rest on the central incisor, acts as a lever arm, causing a belated orthodontic movement of the canine.

• A cantilevered FPD is favored to replace molars and avoid a unilateral RPD, but does not improve masticatory efficiency.

• An alternative to the cantilever FPD could be osseointegrated implants to enhance mastication.

• As osseointegrated implants become more popular, the need for the tooth-supported cantilever FPD may decline but it will remain an alternative treatment if implants are contraindicated for anatomic, medical, economic, or psychologic reasons.

Summary• The current, optimal treatment for replacing missing teeth

is an FPD secured at both ends. • The cantilever is considered a compromise but is preferred

to the RPD, especially for unilateral edentulous dentitions.• The stress distribution in the cantilevered FPD requires the

abutment adjacent to the pontic to have suitable alveolar support, and the preparation of the farthest abutment must be near parallel to prevent dislodgement of the retainer.

• Technical failures are more common when nonvital teeth are abutments, because deterioration of tooth structure can be insidious.

• More occlusal force can also be inadvertently extended to nonvital teeth because their pain threshold is more tolerant.

• Geriatric patients prefer the comfort of a cantilever FPD to an RPD, and less maintenance is required at subsequent appointments.

• With the rapid advancement of osseointegrated implants, the cantilever FPDs may be used sparingly.